In these articles I generally refrain from describing the magnificence and uniqueness of the local landforms but give a broad outline of the formation of the landscapes they are part of, and a little of their history in the hope that these explanations may remove some of the bewilderment about how such natural, often complex wonders originated
The Kimberley is a large regional area of north west Western Australia and fronts the Indian Ocean and the Timor Sea to its west and north, with the Great Sandy Desert to its south and the Northern Territory to the east. Being in tropics means the Kimberley is warm to hot throughout the year with a climatic pattern described as monsoonal with hot and wet summers and cooler dry winters
Two major rivers, the Ord and the Fitzroy arise in the south east Kimberley around Mount Wells (983m) the regions highest point. The Ord tracks east then north via Lake Argyle to Cambridge Gulf north of Wyndham and the Fitzroy flows to the diagonally opposite south eastern corner of the Kimberley and after being joined by several tributaries discharges in King Sound northeast of Derby. There are many other rivers flowing off mounts and ranges on the central Kimberley Plateau and like the Ord and Fitzroy are seasonal, responding the to the wet season for their major flows usually between October to April, with greatly reduced flow volumes during the long, intervening dry season.
Physically the Kimberley is composed of a number of regions but the largest and most prominent is the Plateau that occupies its main northern and western section and forms the majority of the Kimberley’s coastline. This region lacks any large towns but has several smaller ones with many Indigenous centres and outposts. Other than those connected by sealed roads (the “black-top”) most of the smaller communities remain somewhat inaccessible by road during the wet season.
Makeup of the Kimberley Plateau
The rock of Kimberley Plateau began its history over 1,800 million years ago (Ma) when sediments from surrounding mountains were deposited in a large, shallow marine basin with dimensions of at least 450 km north to south and over 400 km east to west. Rivers carrying sandy sediments would have fanned out over the basin and deposited their load forming what is todays Wunaamin Miliwundi Sandstone (formerly referred to as the King Leopold Sandstone) that was built up to depths of more than 800 m in some locations. Following this event that lasted for several million years volcanic lava was extruded over the surface of the sandstone. On cooling this mainly basalt rock varied in depth from 60m deep to over a kilometre. This rock is referred to as the Carson Volcanics. The eruptions were not from typical volcanic cones, but from surface fissures and the lava flowed and filled in hollows and valleys where it became the deepest and remained shallower over higher surfaces of the Sandstone.
Following another pause over many millions of years new mountains had arisen and the basin had continued to slowly sink. The new lot of sediments formed another sandstone to be spread over the Carson Volcanics. This sandstone is referred to as the Warton Sandstones and dating of Zircon crystals from within the sandstone suggest a maximum age for this formation of 1,786 Ma. Two later formation-forming episodes led to the deposition of Elgee Siltstone and over these thick beds of the Pentecost Sandstone.
Two other rock types found in the Kimberly Basin group are those of the Yampi Formation and Hart Dolerite. Unlike the earlier formations Yampi Formation has a distribution limited to the south west of the basin and it this sedimentary formation formed after the Pentecost Sandstone that has become economically important. One of its main rock components is hematite, the iron ore mineral and there has been large scale mining of this on the Yampi Peninsula since 1936. The complex of formations plus the Hart Dolerite is referred to as the Kimberley Series.
Each of the titles allocated to these Formations has its own interesting history and the Pentecost Sandstone for example was named after John Pentecost, the geologist member of a survey team led in 1882 by Michael Durack. The Yampi Formation simply takes its name of the location where the formation is found and the state’s naming authorities have no idea of the origin of the name Mount Hart. The site where the type specimen of Hart Dolerite was collected was Hart Range shown on Alexander Forrest’s chart after he had traversed this area in 1879.
Hart dolerite has a similar composition to the Carson Volcanics basalt but given that dolerite did not reach the surface, it cooled more slowly underground and so developed larger crystals in its makeup. Dolerite is an intrusive rock where the magma, pushed up with great pressure was forced up through joints in the rock and intruded horizontally between the beds of the sedimentary rock where it cooled to form sills. Most of Hart Dolerite is found within the Wunaamin Miliwundi Sandstone and up into the Warton Sandstone. Because of its mineral composition and resultant dark colour, it is hard to distinguish Hart Dolerite when found associated with the Carson Volcanics. Dating of the Dolerite indicates an age of formation around 1795 Ma.
The Effects of Earth Movements.
Most of the Kimberley Basin formations were laid down horizontally and have remained that way even during the Basin’s uplifts to become a plateau. However, the landforms of the Yampi Peninsula in the Plateau’s south western corner, and areas south of this, including the Wunaamin Miliwundi Ranges (originally referred to as the King Leopold Ranges) were initially formed by mountain forming forces called orogenesis. In the south west Kimberley orogenic events were compressive acting mainly form the pushing forces from the south and these occurred about 1000 Ma and again between 670 to 510 Ma ago. These huge forces produced folded mountainous landscapes with many high tops and low valleys and in some cases pushed the more pliable folds over one another and caused faulting in brittle rocks.
Horizontal strata v’s folded strata.
Lower beds in horizontal strata (left side) are not weathered and eroded until those above have been removed. Not only does folding produce hills and valleys (middle image below) but, following weathering and erosion of the high points (right side), several of the ‘lower’ beds are also exposed to weathering and eroding agents.
Another forceful effect transmitted throughout the plateau crated jointing seen particularly in the sandstone formations. Joints ran through the mass of rock rupturing it vertically which combined with the horizontal bedding layers created blocks. The vibrating forces that created the joints is believed to have emanated from orogenesis in both the east and south of the plateau and have created a series of often deep seated joint lines that run from the southeast to the northwest and from the southwest to the northeast. Some joints run for many 100s of metres and a few extend for many 10s of kilometres.
Geologists have also gathered evidence to illustrate that the plateau has had several uplifting episodes the most recent being in the order of 200 Ma and about 50 Ma ago. Each time there is an uplift, weathering and erosion by rivers is rejuvenated and it is these most recent uplifts that have been responsible for the development of many of the landforms we see today.
In some locations the only formation remaining today is the lowest one, the Wunaamin Miliwundi Sandstone, clearly resulting from long periods of weathering and erosion and the removal of those formations originally above it. However, there are locations visible around its seaward edges of the Kimberley Basin, where Pentecost Sandstone and Elgee Siltstone still remain as the upper predominant strata. In other parts of the Plateau geomorphologists who study these landforms note that these are several high spots assessed as being remnants of the Plateau from earlier times.
What weathers the fastest sandstone or basalt?
In general, the Kimberley sandstones are the most durable sedimentary rock with respect to weathering and the siltstone is most susceptible. We are likely therefore to find that most ridges, escarpments and cliffs are formed from sandstones, whereas valleys are more likely to occur where Elgee Siltstone once predominated.
Contrary to common opinion, basalt and dolerite due to their mineral composition weather more rapidly than the sandstone under environmental conditions that have prevailed in the Kimberley. These two igneous rocks have minerals with a high proportion of iron and so chemically weather more rapidly than the somewhat inert sandstone, composed mostly of silicon dioxide.
Should either of these igneous rock types be exposed within or below a sandstone, their rapid weathering weakens the strata since it no longer offers support and the sandstone collapses and may be broken up by the fall.
The Plateau as seen from its coastline
My most recent Kimberley visits have been aboard Coral Expeditions vessels sailing between the Berkeley River and Broome. The cruise itineraries mostly involved experiences along the edge of the Plateau but during the section between Broome and south King Sound we pass Kimberley landscapes of the Dampier Peninsula that are not part of the Plateau. This section also includes the offshore Lacepede Islands.
Along the edge of the Plateau cruises pass many sea cliffs and venture into estuaries and bays. Here it is possible to see the characteristics and relationships between each of these six Plateau formations and the Hart Dolerite especially at sites where they are washed clean and/or devoid of soil and vegetation. Such features include-
- Warton Sandstone cliff-lined estuaries of the Berkeley (see above) and King George rivers,
- visits to sites like Jar Island, Bigge Island and Swift Bay to see rock art from past indigenous clans painted mostly on Wunaamin Miliwundi Sandstone surfaces;
- mangrove lined Porosus Creek with its great biological diversity surrounded by Wunaamin Miliwundi Sandstone cliffs and sills of Hart Dolerite (see above);
- the wave cut underwater plateau of Montgomery Reef and its fascinating tidal patterns;
- the complex of colours, shapes rock types including the Horizontal Waterfalls in Talbot Bay and nearby, Nares Point in Yampi Peninsula (see above).
Read more in ‘Australia’s Kimberley Coast’ by A W (Sandy) Scott.